1. Field of Industrial Utilization
The present invention relates to a two-wave antenna for telephones used in vehicles that includes an antenna element made of two radiating elements connected to both ends of a single phase coil so as to transmit and receive a first wave in the 1.5 GHz band and a second wave in the 0.9 GHz band.
2. Prior Art
FIG. 3 shows an electrical structure of the antenna element used in a conventional two-wave antenna for automobile telephones. In this antenna element 30, two radiating elements 31 and 32 are connected to both ends of a single induction element 33, so that transmission and reception of a first wave in the 440 MHz band and a second wave in the 140 MHz band are accomplished. The effective radiating lengths of the two radiating elements 31 and 32 and the electrical length of the induction element 33 are respectively set at 1/2 .lambda.a where .lambda.a is the wavelength of the first wave. The induction element 33 of this antenna element 30 acts as a phase coil with respect to the first wave in the 440 MHz band and also acts as a loading coil with respect to the second wave in the 140 MHz band. Thus, the current distribution of the first wave in the 440 MHz band is indicated by Ia, and the current distribution of the second wave in the 140 MHz band is indicated by Ib.
In this case, the ratio of the frequency of the first wave to the frequency of the second wave, which is "440/140", is roughly equal to 3. Accordingly, an antenna element of prescribed dimensions made for use in the 440 MHz band can be used "as is" as an antenna element for the 140 MHz band.
Recently, however, there has been a demand for two-wave antennas for telephones used in vehicles which are capable of transmission and reception using a first wave in the 1.5 GHz band and a second wave in the 0.9 GHz band. In response to this demand, there is an antenna element 40 which is obtained by interposing an induction element 43 as a phase coil between two radiating elements 41 and 42 as shown, for example, in FIG. 3(b) so that transmission and reception of a first wave in the 1.5 GHz band can be accomplished. In this antenna element, the current distribution caused by the first wave in the 1.5 GHz band is as indicated by Ic, and good transmission and reception characteristics are obtained. However, the current distribution caused by the second wave in the 0.9 GHz band is indicated by Id, and a so-called "splitting" occurs in the radiation pattern. As a result, good transmission and reception characteristics are not obtained.
An antenna for solving this problem has been proposed, and in this antenna a parasitic antenna element is used. This antenna element is installed next to a dipole antenna so that the parasitic antenna element transmits and receives one wave and the dipole antenna element transmits and receives the other wave. However, this antenna has a complicated structure and lacks reliability. In addition, it has a drawback of high cost.